1. Field of the Invention
The present invention relates to an Orthogonal Frequency Division Multiplexing (OFDM) system and, in particular, to a method and apparatus for transmitting and receiving a preamble having sequence information for an OFDM system.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) is a bandwidth efficient digital modulation technique that has been adopted for a variety of advanced broadcast standards such as Digital Video Broadcasting-Terrestrial (DVB-T). Digital Video Broadcasting-Cable 2 (DVB-C2), a new standard for broadcast transmission over cable, has also selected OFDM as its modulation technique to provide a high degree of efficiency and flexibility.
The DVB-C2 system supports the feature of flexible and dynamic bandwidth allocation by combining various adjacent channels to a single wideband channel. In the case of a single channel implementation, the bandwidth of the DVB-C2 becomes 8 MHz, equal to that of the DVB-C. Accordingly, when N channels are combined, the bandwidth of the DVB-C2 system increases N times compared to the DVB-C. This means that the DVB-C2 has the bandwidth of N×8 MHz.
A comparison will now be made between the channel combinations in the DVB-C and the DVB-C2. In the DVB-C system the number of guard bands increases in proportion to the number of channels combined due to the guard bands required to ensure separation between channels. In an exemplary case of a combination of 4 channels, the total bandwidth available for the data transfer is equal to a value obtained by subtracting 5 guard bands from the entire 24 MHz bandwidth (4×8 MHz).
In contrast, the DVB-C2 system transmits signals using the OFDM which does not require guard bands between channels except for the guard bands at the edges. Accordingly, when 4 channels are combined, the total bandwidth available for the data transfer is equal to a value obtained by subtracting 2 guard bands from the entire 24 MHz bandwidth (4×8 MHz). From the comparison, it is obvious that the DVB-C2 system is superior to the DVB-C system in spectral efficiency.
FIG. 1 is a diagram illustrating a frame format of a conventional DVB-C2 system with a fixed tuning window.
In FIG. 1, two channels 101 and 102 occupy two different frequency bands, and each channel is composed of different broadcast data (i.e. PLP1 105 and PLP2 106 for the first channel 101 and PLP3 107, PLP4 108, and PLP5 109 for second channel 102). The frame starts with preambles 103 and 104 containing control information for the respective channels 101 and 102. In this case, the receiver is tuned to channel 1 101 or channel 2 102 to receive the broadcast data transmitted on the corresponding channel.
In order to improve the resource allocation efficiency, the broadcast data is scheduled over entire channel bandwidth. FIG. 2 is a diagram illustrating a frame format of another conventional DVB-C2 system. In FIG. 2, the bandwidth of each preamble block is identical to a reception bandwidth of 8 MHz as denoted by reference numeral 220. It is noted that the broadcast data 213 is allocated regardless of a boundary of the channels. In this case, the same control information is transmitted within the two preambles 202 and 203. The receiver can align its tuning window 220 to receive the target broadcast data without a need to be aligned with the preambles. Here, the maximum frequency bandwidth of each broadcast data cannot be greater than the minimum reception bandwidth of the receiver (in FIG. 2, 8 MHz).
It can be considered that a preamble block is allocated a bandwidth narrower than the reception bandwidth of the receiver unlike the exemplary case of FIG. 2 in which the bandwidth of each preamble block is identical to the reception bandwidth of the receiver. In this case, the preamble can be received within the reception bandwidth (8 MHz) without being segmented.
FIG. 3 is a diagram illustrating a frame format of another conventional DVB-C2 system. In FIG. 3, the bandwidth of a preamble block is narrower than the reception bandwidth of the receiver. When the bandwidth of a preamble is narrower than the reception bandwidth of the receiver, the receiver can receive a non-segmented preamble block 302 within its reception bandwidth. In this case, however, a specific preamble block 303 is received due to the tuning to the bandwidth of the channel carrying the preamble block 302. This is because the frequency band outside the frequency band 303 is another system bandwidth, e.g. a frequency bandwidth allocated for a communication system other than the DVB-C2.
FIG. 4 is a diagram illustrating a principle of operation of the receiver in the conventional DVB-C2 system when segmented preambles are received in the reception bandwidth. When the reception bandwidth as denoted by reference numeral 420 is aligned to receive the broadcast data (PLP2) as shown in FIG. 4, the preamble information as denoted by reference numeral 401 is received within the reception bandwidth. In this case, since the preamble information is received within two preamble blocks 410 and 411, the information carried by the preamble blocks 410 and 411 must be reordered to obtain the complete information as denoted by reference numeral 430.
Accordingly, in order to obtain the complete control information in the above described conventional DVB-C2 system, the receiver must estimate a boundary frequency 425 and reorder information carried by the preambles based on the boundary frequency 425, thereby processing latency. Particularly, when the boundary frequency is misestimated due to the frequency offset, the receiver is likely to fail to obtain complete control information from the preamble, resulting in reception failure of the data within the entire frame.